Embedded Files
By Ayush Pandey, BS-MS 2020
The rotation of the planet around its axis can generally be of two types: prograde and retrograde. Prograde rotation is referred to when the planet orbits and spins in the same direction, and retrograde rotation is referred to when the planet orbits and spins in opposite directions. In our Solar System, most of the planets are in prograde rotation; they rotate and orbit in a counter-clockwise direction when viewed from the sun's north pole. But Venus and Uranus are the only two planets in retrograde rotation; they orbit in a counter-clockwise direction but rotate in the clockwise direction when viewed from the sun's north pole.
Why only these two planets behave differently from the rest?
When scientists began observing Venus in the late 1960s, they expected it to be pretty stagnant. However, to their surprise, Venus turned out to be quite remarkable, and the most bizarre thing was its retrograde motion. Scientists came up with different hypotheses and theories regarding it. The earliest idea suggested that when our solar system was forming, Venus might have crashed some other planet so hard that it started to spin and rotate in the backward direction (clockwise direction). Still, this approach faced some issues, and the biggest one is that it does not have any physical evidence.
If there was a collision, some sort of remnants should have been produced by the crash, and Venus should have some satellite moons circling or some rings composed of these remnants, but since Venus doesn't have either of these, this is, so the hypothesis was discarded.
After the failure of this theory, scientists began to explore more about the physical properties of Venus, which was possible with spacecraft like Venera and Mariner, and they suggested that Venus was somehow flipped 180 degrees on its axis. So ideally, Venus is still spinning in the prograde rotation, but since it was somehow knocked upside down, it appears to be spinning in a clockwise direction.
Now the question that naturally arises from this theory is that what made this planet flip?
Well, this could happen because of Venus's atmosphere and the mysterious process happening under its interior. Venus is differentiated, i.e., it has layers like core, mantle, and crust. As this planet spins on its axis, there would be friction between the core and the mantle. Moreover, Venus has a thick atmosphere, which experiences strong tidal forces. This hypothesis suggests that at one point in the past, the strong friction between core and mantle and high atmospheric tidal pull could have generated large enough torque destabilizing the planet and flipping it over by almost 180 degrees.
A theory, given by Alex Alemi and David Stevenson from the California Institute of Technology, proposed two significant impacts in Venus's history. The first impact's result was a satellite for Venus. However, after 10 million years, Venus encountered another massive impact on the opposite side of the planet from the first impact, which reversed its rotation. As a result, the satellite it once had spiralled in and collided with the planet itself and got destroyed; it also proved why Venus has no moons.
Another planet that piques the interest of astronomers on account of its eccentric rotational dynamics is Uranus. All the planets spin on their axis, albeit with a bit of tilt. But the Uranus is tilted so much that it rolls rather than spinning! With a tilt of 97 degrees, it has a unique orientation of spin. Similar to the conjecture drawn for Venus's rotation, researchers posit that Uranus had the normal prograde rotation in the early days. But somewhere down the line, it was struck by a heavy celestial body (larger than the size of Earth), tilting this ice giant by 97 degrees and making it rotate in the backward direction.
So one of the pertinent questions about Uranus is why is it lying on its side?
Similar to the conjecture drawn for Venus's rotation, researchers posit that Uranus had the normal prograde rotation in the early days. But somewhere down the line, it was struck by a heavy celestial body (larger than the size of Earth), tilting this ice giant by 97 degrees and making it rotate in the backward direction.
The latest theory, given by Gwenael Boue and Jacques Laskar from the Paris Observatory, shows that Uranus had an enormous moon with 0.1 % of Uranus's mass. The gravitational interaction between the two changed Uranus's axis to tilt the way it is now and caused it to rotate on its side, following retrograde motion. Unfortunately, astronomers and scientists don't have quite as much information about Uranus as we do for Venus. That's because Uranus is far away from Earth—approximately 3 billion kilometers away, and till now, only one spacecraft visited it.
One can't be entirely sure regarding the theories discussed above, but researchers and astronomers are still trying to solve this mystery behind weird rotation. Hopefully, they can arrive at some conclusion regarding this; till then, these planets are just enjoying the attention of this.
Page updated
Google Sites
Report abuse